Debrisoquin sulfate, a postganglionic sympathetic blocking agent, was administered to 28 patients with sustained diastolic hypertension. The total daily dose, which ranged from 20 to 140 mg, averaging 69 mg, was given in divided doses at 6- to 12-hour intervals. A significant reduction in mean blood pressure (MBP) (20 mm Hg or more) was achieved with administration of debrisoquin alone in five of the 25 patients when recumbent and in 12 patients when standing, and on the addition of hydrochlorothiazide for six of the 14 patients when recumbent and 11 patients when standing. Debrisoquin would appear to have definite but limited utility as an antihypertensive agent, affecting primarily orthostatic blood pressure. It produces few unpleasant symptoms and no overt toxic effects upon kidney, liver, or bone marrow. Its ultimate usefulness will be found in combination with other drugs.